This invention relates to a new method of preparing crystalline aluminosilicate molecular sieve compositions with advantageous properties.
Zeolitic materials, both natural and synthetic, are known to have catalytic capabilities for many hydrocarbon processes. Zeolitic materials, typically, are ordered porous crystalline aluminosilicates having a definite structure with cavities interconnected by channels. The cavities and channels throughout the crystalline material generally are uniform in size allowing selective separation of hydrocarbons. Consequently, these materials in many instances are known in the art as "molecular sieves" and are used, in addition to selective adsorptive processes, for certain catalytic properties. The catalytic properties of these materials are affected to some extent by the size of the molecules which selectively penetrate the crystal structure, presumably to contact active catalytic sites within the ordered structure of these materials.
Generally, the term "molecular sieve" includes a wide variety of both natural and synthetic positive-ion-containing crystalline zeolite materials. They generally are characterized as crystalline aluminosilicates which comprise networks of SiO.sub.4 and AlO.sub.4 tetrahedra in which silicon and aluminum atoms are cross-linked by sharing of oxygen atoms. The negative framework charge resulting from the substitution of an aluminum atom for a silicon atom is balanced by the use of, for example, alkali-metal or alkaline-earth-metal cations, ammonium ions, or hydrogen ions.
Prior art developments have resulted in formation of many synthetic zeolitic crystalline materials. Crystalline aluminosilicates are the most prevalent and, as described in the patent literature and in the published journals, are designated by letters or other convenient symbols. Examples of these materials are Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No. 3,130,007), Zeolite ZSM-4 (U.S. Pat. No. 3,578,723), Zeolite ZSM-5 (U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (U.S. Pat. No. 3,709,979), Zeolite ZSM-12 (U.S. Pat. No. 3,832,449), Zeolite NU-1 (U.S. Pat. No. 4,060,590) and others.
Shape selective molecular sieves are known in the art. Generally, these sieves are prepared by crystallizing a mixture of sources for an oxide of silicon, an oxide of aluminum, a metal cation, and organic template. Usually, a specific sieve is characterized by a particular range of compositions, such as may be expressed in terms of mole ratios of oxides, coupled with a definite X-ray diffraction pattern. However, it is known that a substance is defined by its properties and that various catalytic substances which ostensibly have similar compositions and X-ray spectra may differ in catalytic properties due to subtle differences around catalytically-active sites. The present invention demonstrates this principle especially in mordenite-like molecular sieves. Such sieves prepared according to this invention possess advantageous properties over other molecular sieves of a similar type having similar compositions and X-ray spectra, but prepared in different manners. It has been discovered that a series of molecular sieves can be prepared using 2-aminopyridine as an organic template. The crystalline forms, as determined from X-ray diffraction analysis, of these compositions include mordenite-like and ferrierite-like forms and forms exhibited in aluminosilicates identified as ZSM-4 and ZSM-5.
Specifically, mordenite compositions described by this invention selectively hydrodealkylate substituted aromatic compounds compared to conventionally-prepared mordenites. Thus, an alkyl substituted aromatic such as ethylbenzene can be converted to benzene or toluene which may have greater value. For example, in a typical feed stream for a xylene isomerization unit contains 5 to 25% of ethylbenzene. A selective catalyst which would hydrodealkylate ethylbenzene to benzene and toluene in xylene isomerization would have substantial utility.
Molecular sieves characterized as "mordenite" by chemical composition and X-ray spectra are known as naturally occurring materials and as synthesized materials. For example, a conventional mordenite sieve is produced by crystallizing a basic mixture of sodium aluminate and an oxide of silicon without the use of an organic template compound. Such mordenites are described in D. W. Breck "Zeolite Molecular Sieves," John Wiley & Sons, 1974, incorporated by reference herein. U.S. Pat. No. 4,061,717 discloses a method of producing a mordenite molecular sieve using an ionene polymer as an organic template compound in the crystallization of the sieve. U.S. Pat. No. 4,107,195 discloses synthetic mordenite prepared as a by-product using 1,4-butanediamine, ethylenediamine and pyrrolidine as organic templates.
Molecular sieves characterized as "ferrierite" by chemical composition and X-ray spectra are known as naturally occurring materials and as synthesized materials. For example, a conventional ferrierite sieve is produced by crystallizing a basic mixture of sodium aluminate and an oxide of silicon without the use of an organic template compound. Such ferrierites are described in D. W. Breck "Zeolite Molecular Sieves," John Wiley & Sons, 1974, incorporated by reference herein. U.S. Pat. No. 4,000,248 discloses a method of producing a ferrierite molecular sieve using N-methyl pyridinium hydroxide as an organic template compound in the crystallization of the sieve. U.S. Pat. Nos. 4,016,245, 4,107,195 and 4,046,859 disclose the formation of a ferrierite-like material using an organic template derived from ethylenediamine, pyrrolidine or butanediamine, or organometallic 2-(hydroxyalkyl)trialkylaluminum compounds.
Aluminosilicate molecular sieves identified as ZSM-5 are described in U.S. Pat. Nos. 3,702,886 and 4,139,600. Such aluminosilicates are prepared using organic templates such as tetraalkyl ammonium salts, primary alkyl amines and alkylene diamines as described in U.S. Pat. Nos. 4,139,600 and 4,151,189. Aluminosilicate molecular sieve identified as ZSM-4 is described in U.S. Pat. Nos. 3,578,723 and 4,021,447 using organic templates including tetraalkyl ammonium salts, pyrrolidine and choline salts.